#include<stdio.h>
#include<math.h>
int main()
{
	double x=1,y=1,flag=1,sum =0;
	while(fabs(y)>= 1e-6)
	{
		sum += y;
		flag = -flag;
		x += 3;
		y = flag/x;
	}
	printf("%f",sum);
	return 0;
 } 
